A human skin is composed of a epidermis and a dermis in this order from the surface side. On the lower side of the skin is a subcutaneous tissue, such as a fat layer, and muscular tissue. Intracutaneous injection is an injection into the epidermis or the dermis, and subcutaneous injection is an injection into the subcutaneous tissue. The intracutaneous injection and the subcutaneous injection are selected according to the kind of medicinal liquid and the purpose of the injection.
Since the epidermis and the dermis are comparatively thin, it may not always be easy for an unskilled person to achieve the subcutaneous or intracutaneous injection desired. For this reason, injectors have been proposed in which the advancement of the injection needle in the direction of the skin can be limited by a predetermined stopper so as to control the depth of puncture. Examples of these injectors are disclosed in Japanese Patent Publication No. 2648314 and Japanese Application Publication No JP-T-2007-528274.
Though the injector allows regulation of the puncture depth of the injection needle, the injector is unstable during injection and may not maintain a constant puncture depth.
In addition, such an injector requires the implement to be filled with a medicinal liquid prior to injection, which renders the injector unsuited to so-called prefilled injectors. Further, such an injector is relatively complicated in structure.
Since the just-mentioned type of injector is not of the prefilled type, it requires much effort and attention for filling the implement with a proper amount of medicinal liquid accurately, and needs appropriate control of the medicinal liquid separate from the injector. Therefore, with such an injector, for example, it is difficult for a patient who is weak-sighted or who is not deft to inject a medicinal liquid by himself or herself.